[flang] Add semantic check for C1520

As Fortran 2018 C1520, if proc-language-binding-spec with NAME= is
specified, then proc-decl-list shall contain exactly one proc-decl,
which shall neither have the POINTER attribute nor be a dummy procedure.
Add this check.

Reviewed By: klausler

Differential Revision: https://reviews.llvm.org/D127725